The fracture and toughness of woods

Crack propagation in various woods has been examined by scanning electron microscopy, and the observations related to measurements of fracture toughness. It is found that the toughness is related in a simple way to the density of the wood, which is explained by a straightforward model. The apparent fracture toughness of wood for cracks that lie normal to the grain is larger, by a factor of about 10, than that for cracks which propagate parallel to the grain. This difference can be explained in terms of the fracture mechanics of very anisotropic solids.